TO SOME ENGINEERS, a do-it-yourself modular test and measurement system seems to connote a sort of "Wild West" design mentality in which solutions are quickly patched together. They may not be compatible or be able to be managed by a common software platform. In addition, support may be sorely lacking. With the larger test and measurement companies adding modular solutions to their roadmaps, however, many of these concerns are completely unfounded. In addition, standards are quickly being adopted and more widely leveraged in modular approaches. In the microwave and RF market, for example, PXI and its extensionAXIeare commonly at the heart of modular systems.

PCI eXtensions for Instrumentation (PXI) modular instrumentation vows to deliver a rugged, PC-based, and high-performance measurement and automation system. With this open standard, designers can take advantage of the low cost, performance, and flexibility of the latest PC technology. PXI combines standard PC technology from the CompactPCI specification with integrated timing and triggering to deliver a rugged platform with up to a 10X performance improvement over older architectures. (For more information, visit the PXI Systems Alliance.)

According to David A. Hall, National Instruments' Product Manager for RF and Wireless Test, "A wide range of RF and microwave measurementsfrom the basic spectrum sweep to modulation-quality measurements such as error vector magnitude (EVM)are generally processor-intensive and traditionally very slow. Using high-performance multicore CPUs inherent in PC-based instrumentation, many of these measurements can be performed 5X to 10X faster in PXI versus traditional rack-and-stack instrumentation. For any application involving automated testing, even the smallest improvement in measurement speed is a compelling benefit of modular instruments."

Hall continues, "A second unique benefit of modular instrumentation is its use in highly customized measurements in research and development. For example, in MIMO research, we've seen customers build custom MIMO measurement systems by combining multiple downconverters or upconverters into a single PXI chassis to create two, four, and even eight-channel RF signal generators and analyzers. For these customers, the modularity of PXI is not only a big cost savings, but it often allows customers to replace a full rack of instruments with a single PXI chassis."

PXI approaches also translate into lower price points, as they use inexpensive off-the-shelf PC components for the digital portion of the instrument. For an example, Hall points to a mid-range VSA like the NI PXIe-5663E 6.6 GHz RF signal analyzer. Although it retails for just $23,999, this signal analyzer achieves a typical displayed average noise level (DANL) of -158 dBm/Hz at 1 GHz. Hall predicts that we will see widespread industry adoption of PXI as the standard for automating RF and microwave measurements over the next two years.

For its part, Agilent is currently focusing its modular products on both PXI and AXIe. AXIe is a standard based on AdvancedTCA with extensions for instrumentation and test. The AXIe Consortium's goal is to provide an open standard that creates a robust ecosystem of components, products, and systems for generalpurpose instrumentation and semiconductor test. AXIe leverages existing standards from PXI, LXI, and IVI. (To learn more, visit www.axiestandard.org.)

According to Mike Millhaem, RF and Microwave Applications Engineer for Agilent's Modular Products Operation, "PXI has broad acceptance across the industry and provides a good starting point for our RF and microwave modular products. AXIe is a great extension to PXI, as it shares many common elements with PXI and allows for more complicated modules due to larger module size, higher per-slot power, and cooling capabilities. In 2010, Agilent introduced the M9392A, a 26.5-GHz vector signal analyzer in PXI. Going forward, we see continued demand for both traditional box instruments and modular instruments and plan to continue development in both areas."

Despite the sudden and growing rise of PXI test solutions, alternatives like VPX and PCI will continue to be leveraged in mainframe approaches. It is unlikely that one will eclipse the other, as they each offer unique advantages. For example, Don Mulder, VP/General Manager of Anritsu Co.'s Microwave Measurement Division, states, "VPX boards combine the inherent advantages of VMEbus, such as robustness and excellent EMC, with the latest high-bandwidth connector capabilities for high-speed differential signaling over the backplane. They also support wide temperature ranges due to advanced cooling methodologies. Plus, VPX boards are in a small 3U form factor." The VPX (also known as VITA 46) specifications deviate from the traditional connector scheme of VMEbus by merging the latest connector and packaging technology with the latest in bus and serial fabric technology.

On the other hand, Mulder notes, "PCI has a very proven track recordespecially in test instrumentsand it is relatively easy to design instruments with PCI boards. PCI also has low latency, which is one reason it remains popular for instrument control."